Self-Healing Smart Materials. Группа авторов
be calculated as the ratio between the tensile strength results for the healed and original sample according to Equation (3.1) [35–38]:
This relation also works for others mechanical properties such as failure stress, failure strain, energy at break or J-integral at crack initiation [36].
Raman Spectroscopy can be used following the evolution of the three main spectral bands: and C–C stretching (υ C–C = 1,590 cm−1), C−S stretching (υ C–S = 650 cm−1), S−S stretching (υ S−S = 500 cm−1) [35], which are related to the chemical structure of the crosslinked material.
In addition, there are optical methods to characterize the healed sample. Optical microscope and digital cameras are used to analyze the healing process [39] and also scanning electron microscopy (SEM) to study the healing of the scratches [40] as can be seen in Figure 3.4. The figure shows two samples of a compound, which were cut and submitted at different conditions to recover the mechanical properties: room temperature (that take 17 h) and 60 °C (that take 7 h).
Figure 3.4 SEM images to trace the healing process of the scratch. (Reused with permission and modified after Peng et al. [40]).
3.3 Particular Cases in Different Elastomers
3.3.1 Natural Rubber (NR)
Natural Rubber (NR) is obtained from rubber plants through coagulation process being the most used type the Hevea brasiliensis. NR is chemically composed by cis-1,4-polyisoprene which presents exceptional properties like superior tear resistance, high resilience and fatigue resistance.
Some authors have studied the reprocessability of epoxidized natural rubber (ENR), adding different fillers. Cao et al. reported the preparation and characterization of ENR modified by a 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidized cellulose nanocrystals (TOCNs), generating free carboxyl groups, which serve as reinforcing fillers and also as cross-linking agents via epoxy-acid reaction [41]. The linkages between the rubber chains and the reinforcement particles consisted in β-hydroxyl ester, as can be seen in Figure 3.5. The interfacial ester bonds were formed between epoxy groups and carboxyl groups and that was probed by FTIR results.
Figure 3.5 Illustration of the TOCNs cross-linked ENR network based on epoxy-acid reaction, using zinc acetate (Zn(OAc)2) and 1,2-dimethylimidazole (DMI) as additives (Reprinted with permission from Cao et al. [41]).
The covalent rubber network presented high fracture strain (>600%) and tensile stress (>5 MPa). Furthermore, the network with exchangeable β-hydroxyl ester bonds at rubber–TOCNs interface can change the network topology via trans-esterification reactions. The samples achieved up to 80% self-healing efficiency.
Xu et al. developed recyclable and self-healable ENR/citric acid-modified bentonite (CABt) composites (Figure 3.6) [42]. CABt presents numerous carboxyl groups on surface, which react with ENR through exchangeable β-hydroxyl ester linkages, as can be seen in Figure 3.7. Meanwhile, the inherent stickiness of ENR matrix and the low crosslinked network facilitate transesterification reactions of β-hydroxyl ester linkages and chains diffusion, which make ENR/CABt composites recyclable and healable.
Some studies have focused on the DA reactions because of its reversible crosslinking property. Trovatti et al. [43] report the furan-modified NR reversible crosslinking. A comparison between the furan-modified NR 1H-NMR spectra and its retro-DA de-crosslinked rubber gives evidence of the reversibility of the reaction. Tanasi et al. [27] studied the use of a DA reaction to obtain a NR reversibly crosslinked and with self-repair capacity. NR was crosslinked via DA reactions following the procedure shown in Figure 3.8: i) NR reacted with maleic anhydride to produce NR-g-MA; ii) furan moieties were grafted to the rubber generating NR-g-furan; iii) pending furans were crosslinked with a bismaleimide, obtaining thermoreversible bridges which crosslinked NR matrix (NR-DA). Authors explained through phenomenological model that the reversibility of the crosslinks could be achieved by dynamic mechanical analysis complemented with chemical analysis. The results show that the rubber compound present mechanical properties and a crosslinks density comparable whit those of a vulcanized NR with low sulfur content, that represent a healing efficiency greater than 80% at low strain.
Figure 3.6 Schematic diagram of chemical reaction between ENR and CABt (Adapted with permission from Xu et al. [42]).
Figure 3.7 Schematic diagram of hybrid rubber network in cured ENR/CABt composite CABt (Reprinted with permission from Xu et al. [42]).
Figure 3.8. Schematic representation of the chemical reaction between: (a) NR and MA, (b) NR-g-MA and FFA, (c) NR-g-furan and bismaleimide (Adapted with permission from Tanasi et al. [27]).
More recently, Khimi et al. studied the influence of carbon black on the self-healing efficiency of NR and investigated an intrinsic self-healing NR by ionomeric interactions [9]. Compounds using NR were mixed in a conventional laboratory two roll mill, combining with Zinc Oxide, Stearic acid, Zinc thiolate, dicumyl peroxide (DCP) and Carbon Black grade N330 in different amounts. The presence of carbon black generate a network, which contains rubber with different degrees of movement restriction: trapped rubber, occluded rubber and bound rubber (Figure 3.9). These constrained rubber region improves the stiffness and elasticity of the matrix increasing tensile strength. However, in Figure 3.10 it can be seen that the healing efficiency decreased with the addition of carbon black from 98% to approximately 40%. This could be explained because of the lower mobility of the chains rubber for the healing process.
Zhan et al. [44] studied graphene (GE)/NR composites with a conductive segregated network prepared by latex compounding, with focus in electronic devices due to rubber-like conductors are considered as one of the most important components in flexible electronics [45]. Usually, the electrical conductivity of a typical conductive rubber decrease due to the tensile cycles in service can destroy the conductive network. Therefore, the objective of the work was to obtain rubber-like conductors that can recover its original properties by a post-treatment. Samples with 10 phr of GE were made and an electrical conductivity of 2.7 S/m was obtained, with a relatively good flexibility. Then, after 4 tensile cycles and subsequent thermal treatment, the electrical conductivity of the sample increased nearly 2 times than that without treatment, indicating that the network destroyed during the tensile cycles can be healed during the post thermal treatment.
Figure 3.9 Schematic presentation of carbon black filler network (Reprinted with permission from Khimi et al. [9]).